The present invention relates to radiation therapy for the treatment of cancer and the like and, in particular, to a planning method and to an apparatus for radiation therapy using implanted radioactive sources (called seeds).
Prostate permanent implant brachytherapy is a radiation treatment technique in which radioactive sources are implanted directly into the prostate and left in place permanently. Typically 50 to 100 small radioactive sources (“seeds”) are implanted near the tumorous tissue.
The seeds may be a radioactive material adsorbed onto small resin spheres contained within a titanium capsule or on the surface of a silver rod also sealed in titanium. Depending on the radionuclide used, the seeds typically have a half-life of approximately 17 to 60 days providing an average energy of emitted photons of approximately 20 to 35 keV with a commercial source strength of approximately 0.5 mCi. Typical radionuclides used are 125I and 103Pd.
The seeds are of a size, e.g., 0.8 mm in diameter and 4.5 to 5 mm long, so that they may be implanted using a hollow needle. The needle provides a lumen 1.3 to 1.5 millimeter in diameter and about twenty centimeters long into which seeds may be inserted along with spacers controlling their separation. The loaded needle is inserted into the patient and then withdrawn while a plunger ejects the contained seeds.
Placement of the seeds, for example, for use in treatment of the prostate may be done transperineally and the needles are guided by a plate having predrilled holes on a regular grid. The depth of insertion of the needles is guided by an image obtained with a transrectal ultrasonic imaging device. In this way, seeds may be accurately placed at selected regular grid locations.
The locations of the radioactive seeds along the grid are desirably selected to provide the prescribed dose to the diseased tissue of the prostrate while sparing surrounding sensitive critical tissue, for example, the urethra, and rectum. Such placement is aided by treatment planning performed before the implantation of the seeds.
In current practice, treatment planning is largely trial and error based on some simple spacing rules after an inspection of the tumor site by ultrasonic or other imaging techniques. More precise treatment planning may be obtained by a number of well known optimization processes providing mathematical simulation of a dose from a given seed pattern.
Determining the dose produced by a given pattern of seeds is a relatively straightforward process, however, the “backwards” calculation from dose to seed pattern is mathematically difficult. Such problems are typically addressed by stochastic techniques such as “simulated annealing” or by “genetic algorithms”, both that perform repeated forward calculations for many possible seed patterns and then apply an objective function to the dose produced by the pattern to select the best pattern.
A deterministic approach to treatment planning is provided by the “branch and bound” method described, for example, in Treatment Planning for Brachytherapy: An Integer Programming Model, Two Computational Approaches and Experiments With Permanent Prostate Implant Planning, by Lee E. K. et al., Phys. Med. Biol. 44: 145-165 (1999), and An Iterative Sequential Mixed-Integer Approach to Automated Prostate Brachytherapy Treatment Plan Optimization, D'Souza W. D. et al., Phys. Med. Biol. 46: 297-322 (2001).
Operating on high-speed computers, these approaches may take as much as an hour to complete for a typical prostate treatment plan. Such delay requires considerable inconvenience and expense to a patient, either in waiting for the implant during the treatment planning process, or in having to return after the treatment planning session at a later date for the implanting. In the situation where the patient returns at a later date, considerable effort and time is required to realign the patient to the position the patient held during the imaging used for the treatment plan.